Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers, having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ an endless belt-shaped fixing film, a heater disposed inside a loop formed by the fixing film, and a pressing roller pressed against the heater via the fixing film to form a fixing nip between the pressing roller and the fixing film through which the recording medium bearing the toner image passes. As the recording medium passes through the fixing nip, the fixing film heated by the heater and the pressing roller together apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium.
FIG. 1 is a vertical sectional view of a fixing device 20R having such configuration. For example, a pressing roller 106 is pressed against a heater 103 via a fixing film 102 to form a fixing nip N1 between the pressing roller 106 and the fixing film 102 through which a recording medium bearing a toner image passes. Since the toner image formed on the recording medium contacts the fixing film 102 as the recording medium is conveyed through the fixing nip N1, the recording medium tends to adhere to the fixing film 102 due to an adhesive force of heated toner of the toner image formed on the recording medium after it is discharged from the fixing nip N1. Otherwise, the recording medium may be wound around the pressing roller 106 due to its curvature. To address this problem, the technology described below is used to facilitate separation of the recording medium from the fixing film 102 and the pressing roller 106.
For example, the fixing device 20R further includes a heater holder 104 that holds the heater 103 and includes a protrusion 104a protruding toward the pressing roller 106 and contacting the inner circumferential surface of the fixing film 102 so as to prevent the recording medium from adhering to the fixing film 102. A pair of rollers 107 and 108 is disposed downstream from the fixing nip N1 in a conveyance direction of the recording medium to form a post-fixing nip N2 between the rollers 107 and 108. These components of the fixing device 20R are arranged as described below to facilitate separation of the recording medium from the fixing film 102 and the pressing roller 106.
A straight line La connects a downstream edge of the fixing nip N1 in the conveyance direction of the recording medium and the summit of the protrusion 104a. A straight line Lb connects the summit of the protrusion 104a and the post-fixing nip N2. A straight line Ln extends along a sectional line of the fixing nip N1. An angle θ2 formed by the straight line Lb and the straight line Ln is greater than an angle θ1 that is formed by the straight La and the straight line Ln and is greater than 5 degrees. With this configuration, even the moisture-laden recording medium can be conveyed precisely without adhering to the pressing roller 106.
However, the configuration shown in FIG. 1 has a drawback in that since the angle θ1 is greater than 5 degrees, the recording medium, when bearing a color toner image on both sides thereof, may be wound around the pressing roller 106 readily. Additionally, the protrusion 104a protrudes toward the pressing roller 106 from the heater holder 104 that guides the recording medium toward the pressing roller 106 in such a manner that the recording medium is beyond the straight line Ln toward the pressing roller 106. Accordingly, if the length of the fixing nip N1 in the conveyance direction of the recording medium is elongated due to variation in the hardness of the pressing roller 106 and pressure applied from the pressing roller 106 to the fixing film 102, the angle θ1 may increase rapidly, thus rendering the recording medium wound around the pressing roller 106.